Background events

If protons collide, all sorts of particles can be created, not only Z or Higgs bosons! For example a W boson or a top quark can pop-up instead or in addition. These particles also decay immediately after their production, since they are as the Z and Higgs bosons, very heavy.

Collisions can also often lead to showers, or so-called “jets” of particles, without Z, W or top. Events with W, top or jets, which could look like a “signal” event, containing the particle you are searching for (Z, Higgs or others), are referred to as background events.

The cartoon drawing shows how a W- boson can be produced and decay.

This diagram shows the predominant process (quark-gluon interaction) to create W- particles. At least one jet is also created from the up quark. So you can see: A W boson will typically decay to only one charged lepton, not two as for the Z boson. And in addition, there is a neutrino. As we learned above, a neutrino will only be noticed due to missing energy in our energy balance.

Here the W- boson is produced from and anti-up (u) and a down (d) quark, and decays into an electron and an anti-neutrino. This is not the only one way a W boson can be produced and decay, there are many more!

You can compare the decay-products to those of the Z boson, for example. Remember: The Z boson was neutral, and therefore the sum of the charge of the decay-products had to equal 0. However, the W boson is not neutral, it has charge +/- 1. Therefore, the decay-products cannot be a particle-anti-particle pair of charged particles.